The Class II MHC Protein and Tetramer Core will support Research Projects 1-3 and the TDC by providing Class II MHC proteins and tetramers carrying Category A-C agent peptides. The specific aims of the proposed core facility are to generate HLA-DRl-peptide tetramers for use in quantifying and characterizing CD4+ T cells responding to category A-C viruses, to generate purified HLA-DRl-peptide complexes for use in development of microassays to detect CD4+ T cells, and to investigate the production of human class II MHC proteins other than HLA-DR1 for use as purified MHC-peptide complexes and tetramers. These are specialized reagents not available commercially. Production in a core facility will provide economies of scale, standardized production, and reagent testing, and will facilitate production of additional alleles. The ready availability of HLA-DR-peptide proteins and tetramers should greatly enhance the ability of the Research Projects to enumerate, isolate, and characterize virus-specific CD4+ T cell responses, and the ability of the TDC to develop and validate new technology for identification of T cell epitopes and characterization of antigen-specific T cell functional responses.